TITLE: GCN CIRCULAR NUMBER: 38493 SUBJECT: LIGO/Virgo/KAGRA S241102br: continued GRAWITA observations and detection of an optical transient (AT2024adip) DATE: 24/12/09 17:09:59 GMT FROM: Andrea Reguitti at INAF - Osservatorio Astronomico di Brera andreareguitti@gmail.com

A. Reguitti (INAF-OAB / INAF-OAPd), P. D’Avanzo (INAF-OAB), A. Rossi (INAF-OAS), L. Izzo (INAF-OACn / DARK), S. Benetti (INAF-OAPd), R. Brivio (INAF-OAB), S. Campana (INAF-OAB), V. D'Elia (ASI), M. Ferro (INAF-OAB), Y.-D. Hu (INAF-OAB), A. Melandri (INAF-OAR), F. Ragosta (Univ. Napoli / INAF-OACn) et al. on behalf of the GRAWITA collaboration

We continued to carry out optical follow-up observations of the well-localized LIGO/Virgo/KAGRA GW trigger S241102br (LVK Collaboration, GCN Circ. #38043).

Targeted observations carried out with the LBC camera mounted on LBT (Mt. Graham, AZ, USA) on 2024-11-09 at midtime 03:24 UT (~6.4 days after the GW trigger) with the Sloan-r and Sloan-z filters revealed the presence of a bright source located at coordinates RA, Dec = 23:01:50.45, +38:41:39.7" (+/- 0.3") not visible in archival Pan-STARRS images. This source lies on the plane of a bright galaxy seen edge-on, with an offset of about 5" from the host galaxy centre. From preliminary photometry we estimate for this source r=21.2+/-0.2 (AB mag, calibrated against the Pan-STARRS catalogue), confirmed by the subtraction of the template images from the PanSTARRS all-sky survey. Therefore, within the Pan-STARRS limits, suggesting this is a new transient.

Re-inspection of the Asiago Schmidt telescope images (Reguitti et al. 2024, GCN #38417), which includes subtraction of the template images from the PanSTARRS all-sky survey, revealed a possible, low SNR detection of the same source already on 2024-11-02, at r=20.9+/-0.4 AB mag.

Finally, observations carried out with the DOLORES camera mounted on TNG (Roque de los Muchachos, La Palma, Spain) on 2024-12-02 (~ 30 days after the GW trigger) in imaging mode with the Sloan r-band filter revealed the source has clearly faded below the detection threshold at r>22.0 AB mag, which entails a fading of at least 0.8 mag. On the same night we performed also TNG/DOLORES spectroscopy with the LR-B grism: from the narrow Halpha emission of the host galaxy we derive a redshift z = 0.0815, corresponding to a luminosity distance DL = 373 Mpc (assuming H0=69.6, Wm=0.286, WΛ=0.714), consistent with the distance reported for the GW event S241102br. Adopting the aforementioned luminosity distance, at the time of LBT observation the source had an absolute magnitude of Mr=-17.0+/-0.2, corrected for the Galactic extinction (Ar=0.38 mag, Schlafly & Finkbeiner 2011).

This new transient source has been loaded into the Transient Name Server with the official IAU name AT 2024adip.

Further observations and analysis of AT 2024adip is in progress.

We thank the staff at LBT Observatory and TNG telescope for their excellent support, in particular the TNG observers Massimo Cecconi, Filippo Ambrosino, Giovanni Mainella, the LBTO staff Alexander Backer, Steve Allanson, and for LBT-Italy Felice Cusano, Ester Marini.

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